Fabrication of vertical cavity surface emitting laser with current confinement

ABSTRACT

A vertical cavity surface emitting laser having a planar structure, having an implantation or diffusion at the top of the mirror closest to the substrate or at the bottom of the mirror farthest from the substrate, to provide current confinement with the gain region, and having an active region and another mirror formed subsequent to the implantation or diffusion. This structure has an implantation or diffusion that does not damage or detrimentally affect the gain region, and does provide dimensions of current confinement that are accurately ascertained. Alternatively, the implantation or diffusion for current confinement may be placed within the top mirror, and several layers above the active region, still with minimal damage to the gain region and having a well-ascertained current confinement dimension.

This application is a division, of application Ser. No. 08/671,995 filedJun. 28, 1996.

BACKGROUND OF THE INVENTION

The present invention pertains to vertical cavity surface emittinglasers (VCSELs), and particularly to VCSELs having current confinement.More particularly, the invention pertains to VCSELs having refinedcurrent confinement caused by an implant or diffusion not havingunwanted damage in the VCSEL structure.

Several patents address the issue of current confinement. U.S. Pat. No.5,115,442 reveals a structure having a semiconductor quarterwave stackin both mirrors. The entire semiconductor epitaxial structure isdeposited first, followed by a deep proton implant to confine thecurrent. This is a commonly used structure. Its drawbacks include thefact that the top mirror is several microns thick, and therefore theimplant must be so deep that one is limited in how small the currentpath can be made. Since the depth is so large, and there is significantstraggle of implanted ions, the diameter of the current confined regioncannot be made as small as one would like. This makes it more difficultto produce a single mode device and more difficult to keep the currentrequired to reach the threshold for lasing small. In addition, damage isproduced in proximity to the active region by the implant, which couldeventually limit the lifetime of the device. The limit on size restrictsperformance. Furthermore, there are reliability concerns due to theproximity of the implanted region next to the gain region.

A second related U.S. Pat. No. 5,256,596 also provides for currentconfinement using ion implantation, but has a mesa etched before theimplanting, so the implant depth is smaller. In that structure, a buriedimplant is used to provide current confinement. However, the entireepitaxial structure is deposited first, and a mesa must be etched beforeion implant, in order to place the implant at the right depth, since therange of dopant atoms is quite small compared to protons. In fact, onecan wonder whether the structure shown in FIG. 3 of that patent is evenfeasible, since it would require the implant of p- type atoms severalmicrons below the surface. The disadvantages of this approach are thatit results in a non-planar surface, and requires implantation through orclose to the active region, thereby resulting in potential reliabilityproblems.

U.S. Pat. 5,475,701, by Mary Hibbs-Brenner and issued Dec. 12, 1995, ishereby incorporated in this specification by reference.

SUMMARY OF THE INVENTION

This invention consists of a vertical cavity surface emitting laser inwhich the current is confined to the center of the device by the use ofan implant or diffusion in mirror layers close to the active layers ofeither mirror; that is, the implant or diffusion may be placed at thetop of the bottom mirror or at the bottom of the top mirror.

The approach outlined here involves a two step metalorganic chemicalvapor deposition (MOCVD) growth. The first mirror is grown, and thenimplanted or diffused to provide current confinement. Then the remainderof the laser structure, i.e., the remainder of the first mirror, thegain region, and the second mirror, is deposited. The structure remainsplanar, thus facilitating the fabrication of high density arrays. Theimplant or diffusion is shallow (a few tenths of a micron), so thedimensions can be accurately controlled. The implant or diffusion isclearly below the active region, and ions do not need to be implanted ordiffused through the active region. This approach provides a structurefor improved reliability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a is a diagram of a VCSEL having a current confining implant ordiffusion below the active region.

FIG. 1b shows a current confining implant or diffusion above the activeregion in the VCSEL.

FIG. 2a is a diagram of another VCSEL, having a current confiningimplant or diffusion below the active region, that can be integratedwith other electronic circuits.

FIG. 2b shows the VCSEL of FIG. 2a but with the current confiningimplant or diffusion above the active region.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1a illustrates configuration 10 of the structure. In this version,alternating epitaxial layers 14 and 16 for laser 10 are deposited on asubstrate 12 which is doped n- type. On the bottom side of substrate 12is formed a broad area contact 15 (i.e., n- ohmic). A bottom mirror 17,consisting of 26 periods of alternating layers of AlAs 16 and Al_(x)Ga.sub.(1-x) As (x=0.15 is preferred, but x may have any value greaterthan 0.05) 14, all doped n- type, are grown to form a highly reflectingmirror 17. The total number of mirror periods may be greater or lessthan 26, depending on other parameters. At the top of mirror 17, a p-type or electrically insulating dopant 20 is implanted or diffused intop layers 16 and 14 in order to block current flow on the perimeter ofmirror 17, and confine the current to dimension 40. This p- orinsulating dopant may be located between 0 and 10 periods (20 layers)below the first confining layers, but preferably is 2 periods below thefirst confining layer. It is preferable for the depth of implant 20 tobe several tenths of a micron but may range between 0.1 and 2 microns.Dimension 40 may be between 0.1 and 60 microns, but is typically severalmicrons, i.e., 2 to 5 microns. Several more mirror periods (0 to 10) maybe formed on top of the implanted or diffused surface followed by themid-portion of structure 10, which consists of two Al_(x) Ga.sub.(1-x)As (x=0.6) confining layers 24. x may be 0.25 or greater. These layers24 are most likely to be lightly doped, n-type on the layer nearest then-doped mirror, and p-type on the layer nearest the p-type mirror,although there is a possibility that these could be left undoped. Layers24 sandwich a region 22 having three GaAs quantum wells 28, separatedfrom one another and confining layers 24 by four Al_(x) Ga.sub.(1-x) As(x=0.25) barrier layers 26. The number of GaAs quantum wells may be fromone to five. Alternatively, one could potentially have an active region22 without quantum wells, e.g., a region having an emitting number ofabout 0.2 micron thick. On top of confining layer 24 on active region22, a p- type mirror 30 is grown, consisting of 18 periods ofalternating layers of p- AlAs 31 and p- Al_(x) Ga.sub.(1-x) As 32(x=0.15 preferably, but may have any value greater than 0.05). Thenumber of periods may be greater or less than 18, depending on otherparameters. A GaAs contact layer 34 is formed on top of mirror 30. Aproton isolation implant 38 is placed at the perimeter of contact layer34, mirror 30, active region 22 and confining layers 24, to separate onedevice 10 from a like neighboring device on a chip. If a single laserchip 10 were to be made, then it is possible that one could eliminatethis proton implant 38, if the implant or diffusion made on top of then-mirror were to extend all the way to the edge of the chip. Laser 10connections are formed by depositing at least one p- type ohmic contact36 on the top surface of contact layer 34, and a broad area n- typeohmic contact 15 on the back side of wafer substrate 12. The resultingdevice 10 emits laser light in the range of 760 to 870 nanometers (nm).

FIG. 1b shows the same VCSEL structure as FIG. 1a, except that dopant 20is implanted or diffused as an n- type or electrically insulating dopantin layers 31 and 32 of mirror 30, preferably several layers aboveconfining layer 24, to function in blocking current flow from theperimeter of active region 22 and lower mirror 17, and to confine thecurrent flow within dimension 40. Dopant 20 has similar dimensions asimplant or diffusion 20 of FIG. 1a.

FIG. 2a illustrates configuration 50 of the structure wherein bothcontacts of the p-n junction can be made from a top surface therebypermitting integration with electronic circuits or other devices on asemi-insulating substrate. In this version, epitaxial layers 14 and 16for laser 50 are deposited on a semi-insulating substrate 12. A bottommirror 17 has 26 periods (i.e., 52 layers) of alternating layers of AlAs16 and Al_(x) Ga.sub.(1-x) As (x=0.16) 14, of which all can be doped n-type, be entirely undoped, or be undoped except for the last fewperiods. Layers 16 and 14 are grown to form a highly reflecting mirror17. A contact layer 54 of n- doped Al_(x) Ga.sub.(1-x) As (x=0.10 butcould range from 0.0 to 0.20) is formed on the top layer 16 of mirror17. In contact layer 54, a p- type or electrically insulating dopant 20is implanted or diffused in order to block current flow on the perimeterof mirror 17 and confine current flow to dimension 40. Dopant 20 hassimilar dimensions as implant 20 of FIG. 1a. Unlike the description forFIG. 1a, in this case, the p-type or electrically insulating dopantregion cannot extend all the way to the edge of the chip, because itwould then prevent us from making this n-ohmic contact 52. The p-type orelectrically insulating implant or diffused area 20 looks like a ring.Dimension 40 is typically between two and five microns. The top andmid-portions of structure 50 form a mesa on contact layer 54, afteretching. The mid-portion consists of two undoped Al_(x) Ga.sub.(1-x) As(x=0.6 but may have a value of 0.25 or greater) confining layers 24which sandwich a region 22 having three undoped GaAs quantum wells 28,separated from one another and confining layers 24 by Al_(x)Ga.sub.(1-x) As (x=0.25 as preferred value) barrier layers 26. On top ofconfining layer 24 on active region 22, a p- type mirror 30 is grown,consisting of 18 periods of alternating layers of p- AlAs 31 and p-Al_(x) Ga.sub.(1-x) As 32 (x=0.15 but x may be at a value of 0.05 orgreater). A p+ GaAs contact layer 34 is formed on top of mirror 30.Layers 34, 31, 32, 26, 28 and 24 are etched on their perimeters down tothe contact layer to form a mesa on layer 54. Proton isolation implant38 may be inserted at the perimeter of contact layer 34, mirror 30,active region 22, and confining layers 24 of the mesa to isolate currentfrom the edge of the mesa. Device 50 could still be fabricated withoutthis proton implant, though it may be more reliable with it. The protonisolation implant may extend into a portion of contact layer 54 at adepth which is less than the thickness of layer 54. The distance betweenthe inside edges of proton implant is between 10 and 100 microns. Laser50 connections for the p-n junction are formed by depositing at leastone p- type ohmic contact 36 on the top surface of contact layer 34, andat least one n- type ohmic contact 52 on an external surface of contactlayer 54 outside the perimeter of the mesa incorporating active region22 and mirror 30, and also outside the perimeter of the p-type orelectrically insulating implant or diffusion.

FIG. 2b shows the same VCSEL structure with similar dimensions andmaterials as FIG. 1a, except that the dopant 20 is implanted or diffusedas an n- type or electrically insulating dopant in layers 31 and 32 ofmirror 30, preferably several layers (0 to 10 periods, or 0 to 20layers) above confining layer 24, to function in blocking current flowfrom the perimeter of active region 22 and lower mirror 17, andconfining the current flow within dimension 40.

Device 10, 50 can be fabricated by epitaxially depositing an n- typemirror in an OMVPE (Organo-Metallic Vapor Phase Epitaxy) or MBE(Molecular Beam Epitaxy) reactor. The layers of device 10, 50 areremoved from the reactor forming the layers, and photoresist is spunonto wafer 10, 50 and patterned in such a way as to protect the layersat an area for a center 40 of device 10, 50. The p-, n-, or electricallyinsulating type dopant is implanted or diffused in a ring outside theprotected area having diameter 40. Device 10, 50 is placed back in theepitaxial growth reactor, and the remaining layers of the structure aredeposited. After growth of the material, the proton isolation implant38, and n- and p- ohmic contact 15 and 36 depositions, respectively, aremade using normal semiconductor processing techniques. When device 10,50 is operated by applying a forward bias to the p-i-n junction formedby the top p- doped mirror 30, undoped, or lightly doped active region22, and bottom n- doped mirror 17, the current is forced to flow onlythrough unimplanted center 40 of device 10, 50.

In the present invention, which has advantages over the above-noted U.S.Pat. No. 5,115,442, the depth of the p- n-, or electrically insulatingtype implant or diffusion need only be a few tenths of a micron but mayrange from 0.1 to 2 microns. Therefore, the diameter 40 of theunimplanted or non-diffused region can be kept small to several microns,but may range from 0.1 to 60 microns, with the realization of needingonly a very low current to reach lasing threshold, in the cases whenthis dimension is kept to just a few microns. In addition, the damagedue to implant 20 is kept away from the active region 22 of laser 10 and50, and thus increases device reliability.

This invention provides advantages over the structure disclosed in theabove-noted U.S. Pat. No. 5,256,596. Since the epitaxial growth iscarried out in two steps, with confining implant or diffusion 20performed after the first growth, one need only implant or diffuse a fewtenths of a micron. In the case of an implant, this limits the energiesrequired, again allowing tighter control of dimensions, and eliminatingthe need for a mesa etch before the implant. That mesa etch exposes thevery reactive AlAs layers 31 in top mirror 30, which would affectreliability. The lower implant 20 energies limit implant damage andmagnitude of the implant straggle. In addition, by keeping implant 20several periods above or below the active region 22, it keeps thereliability limiting implant away from the active layers of the laser.

Other configurations of the device would include the growth of a p- typemirror 17 first, with an n- type or electrically insulating implant ordiffusion 20, followed by the active region 22 and an n- type mirror 30.In addition, InGaAs quantum wells 28 can be used for emission in therange of 870-1000 nm. In that case, light can be emitted from either thetop or bottom surface of laser 10 or 50. Other materials can be used,such as the AlGaInP material system which results in a laser 10 or 50emitting in the range 630-700 nm, or the InGaAsP material system for adevice 10 or 50 emitting near 1.3 microns. Even in the case of thelasers emitting at 760-870 nm, the various compositions mentioned in thedescriptions above can be varied, i.e., "x" compositions in the mirrormight vary from 0.05 to 0.3, or the confining layer "x" compositionsmight vary from 0.4 to 0.8 at the mirrors and from 0.1 to 0.5 betweenthe quantum wells.

We claim:
 1. A method for making a vertical cavity surface emittinglaser having refine d current confinement, comprising:forming a firstcontact layer on a first surface of a substrate; forming a first mirrorhaving a first surface on a second surface of the substrate; implantingor diffusing a dopant at the periphery of and in the first mirror, theperiphery having an inside dimension and the implanting having a depthdimension; forming an active region on the second surface of the firstmirror; forming a second mirror on the active region; and forming asecond contact layer on the second mirror.
 2. The method of claim 1wherein the periphery of the dopant in the second surface of the firstmirror confines current to a center of the vertical cavity surfaceemitting laser according approximately to the inside dimension.
 3. Themethod of claim 2, wherein:the depth dimension is between 0.1 and twomicrons; and the inside dimension of the periphery is between 0.1 and 60microns.
 4. The method of claim 2 further comprising forming a protonimplant at a periphery of the second mirror and the active region. 5.The method of claim 2 wherein the active region has at least one quantumwell.
 6. A method for making a vertical cavity surface emitting laser,comprising:forming a first mirror having a first surface on a substrate;forming a first contact layer having a first surface on a second surfaceof the first mirror; implanting or diffusing a dopant through a secondsurface of the first contact layer, the dopant having a periphery withinside and outside dimensions and having a depth dimension; forming anactive region on the second surface of the first contact layer; forminga second mirror having a first surface on the active region; forming asecond contact layer having a first surface on a second surface of thesecond mirror; etching the second contact layer, the second mirror andthe active region, and forming a mesa of the second contact layer, thesecond mirror and the active region, situated on the first contactlayer; and forming a proton implant into the second contact layer, thesecond mirror and the active region, at a periphery of the mesa, theperiphery having an inside dimension.
 7. The method of claim 6 whereincurrent of the vertical cavity surface emitting laser is confinedapproximately within the inside dimension of the periphery of the firstimplant or diffusion of the dopant.
 8. The method of claim 7 wherein:thedepth dimension of the implanting or diffusion of the dopant into thesecond surface of the first contact layer is between 0.1 and 2 microns;and the inside dimension of the periphery of the implanting or diffusionof the dopant into the second surface of the first contact layer isbetween 0.1 and 60 microns.
 9. The method of claim 6 wherein the activeregion has at least one quantum well.
 10. A method for making a verticalcavity surface emitting laser having refined current confinement,comprising:forming a first contact layer on a first surface of asubstrate; forming a first mirror having a first surface on a secondsurface of the substrate; forming an active region on a second surfaceof the first mirror; forming a first portion of a second mirror having afirst surface on the active region; implanting or diffusing a dopant atthe periphery of and in the first portion of the second mirror, theperiphery having an inside dimension and the implanting or diffusinghaving a depth dimension; forming a second portion of the second mirrorhaving a first surface on a second surface of the first portion of thesecond mirror; and forming a second contact layer on a second surface ofthe second portion of the second mirror.
 11. The method of claim 10wherein the periphery of the dopant in the second surface of the firstportion of the second mirror confines current to a center of thevertical cavity surface emitting laser according approximately to theinside dimension.
 12. The method of claim 11, wherein:the depthdimension is between 0.1 and two microns; and the inside dimension ofthe periphery is between 0.1 and 60 microns.
 13. The method of claim 11further comprising forming a proton implant at a periphery of the firstand second portions of the second mirror, and the active region.
 14. Themethod of claim 11 wherein the active region has at least one quantumwell.
 15. A method for making a vertical cavity surface emitting laser,comprising:forming a first mirror having a first surface on a substrate;forming a first contact layer having a first surface on a second surfaceof the first mirror; forming an active region on the second surface ofthe first contact layer; forming a first portion of a second mirrorhaving a first surface on the active region; implanting or diffusing adopant in the first portion of the second mirror, the dopant having aperiphery with inside and outside dimensions and having a depthdimension; forming a second portion of the second mirror having a firstsurface on a second surface of the first portion of the second mirror;forming a second contact layer having a first surface on a secondsurface of the second portion of the second mirror; etching the secondcontact layer, the first and second portions of the second mirror andthe active region, and forming a mesa on the second contact layer, thefirst and second portions of the second mirror, and the active region,situated on the first contact layer; and forming a proton implant intothe second contact layer, the first and second portions of the secondmirror and the active region, at a periphery of the mesa, the peripheryhaving an inside dimension.
 16. The method of claim 15 wherein currentof the vertical cavity surface emitting laser is confined approximatelywithin the inside dimension of the periphery of the first implant ordiffused area of the dopant.
 17. The method of claim 16 wherein:thedepth dimension of the implanting or diffusion of the dopant into thesecond surface of the first portion of the second mirror is between 0.1and 2 microns; and the inside dimension of the periphery of theimplanting or diffusion of the dopant into the second surface of thefirst contact layer is between 0.1 and 60 microns.
 18. The method ofclaim 15 wherein the active region has at least one quantum well.